Ethylene oxide and other olefin oxides are important industrial chemicals used as a feedstock for making such chemicals as ethylene glycol, propylene glycol, ethylene glycol ethers, ethylene carbonate, ethanol amines and detergents. One method for manufacturing an olefin oxide is by olefin epoxidation, that is the catalyzed partial oxidation of the olefin with oxygen yielding the olefin oxide. The olefin oxide so manufactured may be reacted with water, an alcohol, carbon dioxide, or an amine to produce a 1,2-diol, a 1,2-diol ether, a 1,2-carbonate or an alkanol amine. Such production of a 1,2-diol, a 1,2-diol ether, a 1,2-carbonate or an alkanol amine is generally carried out separately from the manufacture of the olefin oxide, in any case the two processes are normally carried out in separate reactors.
In olefin epoxidation, a feed containing the olefin and oxygen is passed over a bed of catalyst contained within a reaction zone that is maintained at certain reaction conditions. A commercial epoxidation reactor is generally in the form of a shell-and-tube heat exchanger, in which a plurality of substantially parallel elongated, relatively narrow tubes are filled with shaped catalyst particles to form a packed bed, and in which the shell contains a coolant. Irrespective of the type of epoxidation catalyst used, in commercial operation the internal tube diameter is frequently in the range of from 20 to 40 mm, and the number of tubes per reactor may range in the thousands, for example up to 12,000.
Olefin epoxidation is generally carried out with a relatively low olefin conversion and oxygen conversion. Recycle of unconverted olefin and oxygen is normally applied in order to enhance the economics of the process. Generally the feed additionally comprises a large quantity of so-called ballast gas to facilitate operation outside the explosion limits. Ballast gas includes saturated hydrocarbons, in particular methane and ethane. As a consequence, recycling generally involves the handling of large quantities of process streams, which includes the unconverted olefin, unconverted oxygen and the ballast gas. The processing of the recycle stream as normally applied in an olefin epoxidation plant is also fairly complex, as it involves olefin oxide recovery, carbon dioxide removal, water removal and re-pressurizing. The use of ballast gas not only contributes to the cost of processing, it also reduces the epoxidation reaction rate.
The epoxidation catalyst generally contains the catalytically active species, typically a Group 11 metal (in particular silver) and promoter components, on a shaped carrier material. Shaped carrier materials are generally carefully selected to meet requirements of, for example, strength and resistance against abrasion, surface area and porosity. The shaped carrier materials are generally manufactured by sintering selected inorganic materials to the extent that they have the desired properties.
During the epoxidation, the catalyst is subject to a performance decline, which represents itself by a loss in activity of the catalyst and selectivity in the formation the desired olefin oxide. In response to the loss of activity, the epoxidation reaction temperature may be increased such that the production rate of the olefin oxide is maintained. The operation of commercial reactors is normally limited with respect to the reaction temperature and when the applicable temperature limit has been reached, the production of the olefin oxide has to be interrupted for an exchange of the existing charge of epoxidation catalyst for a fresh charge.
It would be of great value if improved epoxidation processes and improved epoxidation reactors would become available.